Here is the summary on how to watch those last few remaining HD-DVDs on your Apple TV or other IOS devices if you have a MAC... surprisingly for just the cost of the HD-DVD drive:
1. Purchase an XBOX 360 HD DVD drive off of eBay. This drive plugs right into your Mac's USB port and is immediately recognized in OS X Mavericks. This drive will play CDs, DVDs, and HD-DVDs. No Blu-rays.
2. Download and install MakeMKV
3. Download and install Handbrake
4. Pop-In the HD-DVD. The DVD player app will run and indicate that the disc is not recognized. Close the application. To my knowledge there is no way to actually watch the HD-DVD in its native format on the Mac.
5. Open MakeMKV. It will access the drive and it should recognize the HD-DVD and display the HD-DVD icon. Click File > Open Disc and it will process the titles on the drive. A list of titles will be presented. Select the ones you want (there are other posts to help you figure those out). Click the MakeMKV button, and after a few hours you will have a digital version of your HD-DVD. This file is huge... basically a copy of the HD-DVD content... like 11GB. MakeMKV does provide an option to stream the HD-DVD content to another device - you could do this - but who has time to stream every movie?
6. Open HandBrake. Open the MKV file. Select the AppleTV 3 device - which will save the AC3 and stereo soundtracks. When complete, this will produce an M4V file that is about half the size of the MKV. Unlike an MP4 file, the M4V file stores chapter information and can support the AC3 (Dolby Digital) audio track. This format will give you the highest audio quality when played on your AppleTV - assuming it is connected to a receiver/pre-amp.
7. Import the M4V file to iTunes and enable Home Sharing, and you will be able to view all of your old HD-DVDs on your Apple TV and other devices in excellent resolution. Note that technically the resolution is lower than the original since it has gone through all the processing.
Hope this helps!